1. Technical Field
The disclosed embodiments relate to amplifiers and impedance matching.
2. Background Information
In radio transmitters of mobile communication devices such as cellular telephones, a driver amplifier on a first integrated circuit is often made to drive a power amplifier on another integrated circuit. The power amplifier in turn drives an antenna such that a radio frequency signal is radiated from the antenna.
FIG. 1 (Prior Art) is a block diagram of a typical circuit. Driver amplifier 1 is a part of a first integrated circuit 2. The particular driver amplifier illustrated includes two stages 3 and 4. Each stage includes a plurality of cells. If the driver amplifier is amplify with more power gain, then more cells of each stage are enabled. If the driver amplifier is to amplify with less power gain, then fewer cells of each stage are enabled. The driver amplifier supplies the output signal onto an output terminal 5, and through a matching network 6 of discrete components, and to an input terminal 7 of a second integrated circuit 8. Second integrated circuit 8 includes a power amplifier 9 that receives the signal from input terminal 7, amplifies the signal, and outputs an amplified version of the signal onto output terminal 10 and to antenna 11. To achieve low distortion and optimal power transfer, the output impedance of driver amplifier 1 should be such that impedance matching occurs at terminal 7 at the input of the power amplifier. Commercially available power amplifier integrated circuits may, for example, have relatively constant input impedances of fifty ohms over the frequency band of the signals being amplified. Unfortunately, as the power of the driver amplifier is decreased due to using fewer and fewer cells in the driver amplifier, the output impedance of the driver amplifier changes. In such a situation, the output impedance looking into terminal 5 may increase or decrease depending on a number of factors. Regardless of whether the output impedance increases or decreases, the change in output impedance may lead to an undesirable impedance mismatch at terminal 7, and hence may lead to distortion in the amplifier.
FIG. 2 (Prior Art) is a chart that illustrates an example of how the impedance matching at terminal 7 may change depending on the number of cells used in the driver amplifier. The cells in the two stages of driver amplifier 1 are identified in FIG. 1 with reference numerals 13 and 14. The horizontal scale from one to sixteen represents the number of the sixteen cells 14 that are enabled and used in the second stage 4 of driver amplifier 1. The quantity VSWR (Voltage Standing Wave Ratio) on the vertical axis of FIG. 2 is considered a measure of mismatch. The VSWR at terminal 7 should be 2:1 or less, yet at low power levels the VSWR is much higher. This is undesirable and may result in undesirable distortion being introduced into the signal output onto antenna 11.
FIG. 3 (Prior Art) is a diagram of a circuit employed to address the problem illustrated in FIG. 2. The circuit includes a programmable matching network 15. Programmable matching network 15 is usable to change the output impedance of the driver amplifier 1 (the impedance looking into output terminal 5). As an example, for operating frequencies in the range of 1.5 to 2.0 gigahertz, capacitor 16 may have a capacitance of tens of picofarads. As the power gain of the driver amplifier changes and the number of cells used changes, the resistance of variable resistor 17 is changed to maintain a substantially constant impedance looking back from terminal 7. The impedance mismatch at terminal 7 of FIG. 2 between the driver amplifier and the power amplifier is reduced or eliminated. Unfortunately, the circuit of FIG. 3 may only be suitable for operation for signals in a single frequency band. It may, however, be required that the mobile communication device of which the transmitter is a part be operable in multiple frequency bands.
FIG. 4 (Prior Art) is a diagram of a circuit topology usable in applications in which the transmitter is to be operable in multiple frequency bands. Two separate driver amplifier/matching network/power amplifier chains 18 and 19 are employed. One chain is impedance matched for operation at frequencies of the first frequency band, whereas the other chain is impedance matched for operation at frequencies of the second frequency band. An output multiplexer 20 is provided to couple the antenna 11 to the output of the appropriate chain. The circuit of FIG. 4 is, however, undesirably large and expensive in that two separate sets of driver amplifiers and matching networks and power amplifiers are used.
FIG. 5 (Prior Art) is a diagram of a circuit operable in multiple frequency bands that does not suffer from the redundant circuitry of the circuit of FIG. 4. The matching network 21 that is coupled to output node 12 of driver amplifier 1 actually involves two capacitor and resistor impedance matching circuits 22 and 23. The appropriate one of the impedance matching circuits for the frequency band of operation is coupled to the output of driver amplifier 1 by opening and closing the appropriate ones of switches 24 and 25. Each of the capacitors 26 and 27 of matching network 21 may, for example, be large and may have a capacitance in the tens of picofarads. A considerable amount of die space may be consumed realizing these capacitors. For example, if the circuit of FIG. 5 is to be operable to amplify either a 2.0 gigahertz signal in a first frequency band (2.0 gigahertz plus or minus fifty megahertz) or a 1.5 gigahertz signal in a second frequency band (1.5 gigahertz plus or minus fifty megahertz), then capacitors 26 and 27 may have capacitances of approximately thirty picofarads and twelve picofarads.